1. Sheet Silicates Abundant and common minerals throughout upper 20 km of crust Abundant and common minerals throughout upper 20 km of crust Felsic to intermediate igneous, metamorphic, and sedimentary rocks Felsic to intermediate igneous, metamorphic, and sedimentary rocks All are hydrous All are hydrous Contain H Contain H Bonded to O to form OH- Bonded to O to form OH- Z/O ratio of 2/5 Z/O ratio of 2/5 2 Major groups: Micas & Clays 2 Major groups: Micas & Clays

2. Groupings Based on structure Based on structure Two kinds of “layers” within the “sheets” Two kinds of “layers” within the “sheets” “T” layers – tetrahedral layers “T” layers – tetrahedral layers Tetrahedral coordination of Si and Al Tetrahedral coordination of Si and Al “O” sheets – octahedral layers “O” sheets – octahedral layers Octahedral coordination of mostly Al and Mg, occasionally Fe Octahedral coordination of mostly Al and Mg, occasionally Fe

3. T and O layers bonded to form sheets T and O layers bonded to form sheets The sheets are repeated in vertical direction The sheets are repeated in vertical direction The spaces between the sheets may be: The spaces between the sheets may be: Vacant Vacant Filled with interlayer cations, water, or other sheets Filled with interlayer cations, water, or other sheets Primary characteristic - basal cleavage Primary characteristic - basal cleavage Single perfect cleavage Single perfect cleavage Occurs because bonds between sheets are very weak Occurs because bonds between sheets are very weak

4. Construction of T-O-T Sheets Octahedral layers: Octahedral layers: Two planes of OH - anionic groups Two planes of OH - anionic groups Cations are two types: Cations are two types: Divalent (Fe 2+ or Mg 2+ ) Divalent (Fe 2+ or Mg 2+ ) Trivalent (Al 3+ or Fe 3+ ) Trivalent (Al 3+ or Fe 3+ ) Mg and Al most common Mg and Al most common

5. Divalent cations fill 3 of 3 sites Divalent cations fill 3 of 3 sites Form trioctahedral sheets Form trioctahedral sheets Ideal formula is Mg 3 (OH) 6 Ideal formula is Mg 3 (OH) 6 This formula is brucite This formula is brucite A hydroxide, not a silicate mineral A hydroxide, not a silicate mineral All sites filled with divalent cations Charge neutral

6. Trivalent cations fill 2 of 3 sites Trivalent cations fill 2 of 3 sites Form dioctrahedral sheets Form dioctrahedral sheets Ideal formula is Al 2 (OH) 6 Ideal formula is Al 2 (OH) 6 Mineral called gibbsite Mineral called gibbsite A hydroxide, not silicate mineral A hydroxide, not silicate mineral 2/3 of sites filled with trivalent cations Charge neutral

7. Tetrahedral sheets Tetrahedral sheets Sheets of tetrahedrally coordinated cations Sheets of tetrahedrally coordinated cations Formula represented by Z 2 O 5 : Z/O = 2/5 Formula represented by Z 2 O 5 : Z/O = 2/5 Z usually Si 4+, Al 3+, less commonly Fe 3+ Z usually Si 4+, Al 3+, less commonly Fe 3+ Symmetry of rings is hexagonal Symmetry of rings is hexagonal Symmetry of sheet silicates is close to hexagonal Symmetry of sheet silicates is close to hexagonal Depends on arrangement of stacking Depends on arrangement of stacking Fig. 11-2

8. Tetrahedron are meshes of 6-fold rings Tetrahedron are meshes of 6-fold rings Three basal oxygen on each tetrahedron shared by adjacent tetrahedron Three basal oxygen on each tetrahedron shared by adjacent tetrahedron The fourth, unshared oxygen is the apical oxygen The fourth, unshared oxygen is the apical oxygen Tetrahedral layers are two oxygen thick Tetrahedral layers are two oxygen thick

9. Fig. 13-1 Tetrahedral sheet composition is Si 2 O 5 2- Tetrahedral sheet composition is Si 2 O 5 2- May have Al 3+ or Fe 3+ substitute for Si 4+ May have Al 3+ or Fe 3+ substitute for Si 4+ Increases net negative charge Increases net negative charge

10. Tetrahedral and octahedral sheets always joined Tetrahedral and octahedral sheets always joined Apical oxygen of tetrahedral sheets formed part of octahedral sheets Apical oxygen of tetrahedral sheets formed part of octahedral sheets Apical oxygen replaces one of the OH- in the octahedral sheets Apical oxygen replaces one of the OH- in the octahedral sheets Sheets joined in two ways Sheets joined in two ways TO layers, called 1:1 layer silicates TO layers, called 1:1 layer silicates TOT layers, called 2:1 layer silicates TOT layers, called 2:1 layer silicates

11. T layer on top OH in middle of rings Al 3+ (dioctahedral) or Mg 2+ (trioctahedral) Basal Oxygen (an example of 1:1 layer type) Fig. 13.1

12. 1:1 layer summary Consists of 3 planes of anions Consists of 3 planes of anions One plane is basal plane of shared tetrahedral oxygen One plane is basal plane of shared tetrahedral oxygen Other side is the OH- anionic group of the octahedral sheet Other side is the OH- anionic group of the octahedral sheet Middle layer is the OH- anionic group with some OH- replaced by oxygen Middle layer is the OH- anionic group with some OH- replaced by oxygen

13. 1:1 layering in phyllosilicates OH- only OH- + oxygen Oxygen only Al 2 Si 2 O 5 (OH) 4 = kaolinite, dioctahedral 1:1 sheet silicate Mg 3 Si 2 O 5 (OH) 4 = serpentine, trioctahedral, 1:1 sheet silicate

14. 2:1 layer silicates 2 tetrahedral layers on both sides of octahedral layer 2 tetrahedral layers on both sides of octahedral layer TOT structure has 4 layers of anions TOT structure has 4 layers of anions Both sides (outermost) are planes of basal, shared oxygen Both sides (outermost) are planes of basal, shared oxygen Middle planes contain original OH- from octahedral layers and apical oxygen from tetrahedron Middle planes contain original OH- from octahedral layers and apical oxygen from tetrahedron

15. 2:1 layering in phyllosilicates Oxygen only OH- + oxygen Oxygen only OH- + oxygen Al 2 Si 4 O 10 (OH) 2 = Pyrophyllite, dioctahedral 2:1 sheet silicate Mg 3 Si 4 O 10 (OH) 2 = Talc, trioctahedral 2:1 sheet silicate

16. How are sheets stacked? I. 1:1 layer I. …T-O…T-O…T-O… II. 2:1 layer I. …T-O-T…T-O-T…T-O-T… II. c…T-O-T…c…T-O-T…c…T-O-T…c… III. O…T-O-T…O…T-O-T…O  Four types of layers, each dioctahedral or trioctahedral  Each one may be dioctahedral or trioctahedral Kaolinite (dioctahedral) Serpentine (trioctahedral) Pyrophyllite (dioctahedral) Talc (trioctahedral)

17. 1:1 layer silicates Kaolinite and Serpentine Kaolinite and Serpentine Bonding between sheets very weak Bonding between sheets very weak Electrostatic bonds – van der Waals and hydrogen Electrostatic bonds – van der Waals and hydrogen Results in very soft minerals Results in very soft minerals Thickness of TO layers around 7 Å Thickness of TO layers around 7 Å

18. C unit cell dimension about 7 Å Fig. 13.3

19. 2:1 layer silicates Unit structure is repeating TOT layers, two ways: Unit structure is repeating TOT layers, two ways: (1) TOT layers can be electrically neutral (1) TOT layers can be electrically neutral Nothing in between sheets van der Waal forces Nothing in between sheets van der Waal forces Pyrophyllite & Talc Pyrophyllite & Talc (2) substitution in TOT layers can give a net charge (2) substitution in TOT layers can give a net charge Most common substitution is Al 3+ for Si 4+ in tetrahedral layers Most common substitution is Al 3+ for Si 4+ in tetrahedral layers Charge balance maintained with substitution between the sheets Charge balance maintained with substitution between the sheets

20. TOT structure TOT structure If there is only Si 4+ in T layers (no Al 3+ or Fe 3+ ) If there is only Si 4+ in T layers (no Al 3+ or Fe 3+ ) Electrically neutral, no interlayer cations Electrically neutral, no interlayer cations TOT layers weakly bonded by van der Waal and hydrogen bonds TOT layers weakly bonded by van der Waal and hydrogen bonds Soft (Talc), greasy feel Soft (Talc), greasy feel

21. C unit cell dimension about 9 to 9.5 Å Nothing in interlayer site Fig. 13.3

22. c…T-O-T…c…T-O-T…c c…T-O-T…c…T-O-T…c These are the mica minerals These are the mica minerals Also less common are “brittle micas” Also less common are “brittle micas” Structure is TOT layers with 1 out of 4 tetrahedral sites occupied by Al 3+ Structure is TOT layers with 1 out of 4 tetrahedral sites occupied by Al 3+

23. Micas Micas Al/Si ratio in the tetrahedral layer is 1/3 Al/Si ratio in the tetrahedral layer is 1/3 Dioctahedral TOT layer = Al 2 (AlSi 3 O 10 )(OH) 2 1- Dioctahedral TOT layer = Al 2 (AlSi 3 O 10 )(OH) 2 1- Remember Pyrophyllite: Al(Si 2 O 5 )(OH) 2 Remember Pyrophyllite: Al(Si 2 O 5 )(OH) 2 Trioctahedral TOT layer = Mg 3 (AlSi 3 O 10 )(OH) 2 1- Trioctahedral TOT layer = Mg 3 (AlSi 3 O 10 )(OH) 2 1- Remember Talc: Mg 3 (Si 2 O 5 )(OH) 2 Remember Talc: Mg 3 (Si 2 O 5 )(OH) 2 Negative charge balance by large monovalent cation, usually K + Negative charge balance by large monovalent cation, usually K + Bonds are ionic, fairly strong, harder minerals Bonds are ionic, fairly strong, harder minerals

24. C unit cell dimension about 9.5 to 10 Å K + in interlayer site

25. Dioctahedral mica – muscovite Dioctahedral mica – muscovite KAl 2 (AlSi 3 O 10 )(OH) 2 KAl 2 (AlSi 3 O 10 )(OH) 2 Trioctahedral mica – Phlogopite Trioctahedral mica – Phlogopite KMg 3 (AlSi 3 O 10 )(OH) 2 KMg 3 (AlSi 3 O 10 )(OH) 2

26. Brittle Micas Similar to micas, but more Al 3+ substitution Similar to micas, but more Al 3+ substitution Charge balanced by Ca 2+ Charge balanced by Ca 2+ Margarite – half of tetrahedral sites have Al 3+ substitution Margarite – half of tetrahedral sites have Al 3+ substitution Clintonite – ¾ of tetrahedral sites have Al 3+ substitution Clintonite – ¾ of tetrahedral sites have Al 3+ substitution

27. Margarite Margarite Dioctahedral Dioctahedral CaAl 2 (Al 2 Si 2 O 10 )(OH) 4 CaAl 2 (Al 2 Si 2 O 10 )(OH) 4 Clintonite Clintonite Trioctahedral Trioctahedral CaMg 2 Al(Al 3 SiO 10 )(OH) 2 CaMg 2 Al(Al 3 SiO 10 )(OH) 2 Now charge balance in part from Al substitution in octahedral layer Now charge balance in part from Al substitution in octahedral layer

28. …O…T-O-T…O…T-O-T…O… …O…T-O-T…O…T-O-T…O… Most common members are in the chlorite group Most common members are in the chlorite group Structure like Talc, but with brucite (Mg 3 (OH) 6 ) interlayer Structure like Talc, but with brucite (Mg 3 (OH) 6 ) interlayer T layers with small negative charge T layers with small negative charge Substitute small amounts of Al 3+ for Si 4+ Substitute small amounts of Al 3+ for Si 4+ O layers often have net positive charge O layers often have net positive charge Substitute Al 3+ or Fe 3+ for divalent cations Substitute Al 3+ or Fe 3+ for divalent cations Minerals harder than expected Minerals harder than expected

29. C unit cell dimension about 14 Å TOT layers have slight negative charge, substitute Al 3+ for Si 4+ O layers often have net positive charge Fig. 13.3 Some Al 3+ for Si 4+ Some Al 3+ for Mg 2+

30. Varieties of sheet silicates TO structures TO structures Serpentine (var. Antigorite, Chrysotile, Lizardite) Serpentine (var. Antigorite, Chrysotile, Lizardite) All are trioctahedral All are trioctahedral Trioctahedral sheets, a = 5.4 Å; b = 9.3 Å Trioctahedral sheets, a = 5.4 Å; b = 9.3 Å Tetrahedral sheets, a = 5 Å; b = 8.7 Å Tetrahedral sheets, a = 5 Å; b = 8.7 Å Mismatched size leads to variations Mismatched size leads to variations

31. Fig. 13-5 Chrysotile (curved tubes) Antigorite (reversed direction) Lizardite (distorted tetrahedral mesh)

32. Clay Minerals Clay has two meanings: Clay has two meanings: Particles < 1/256 mm, or 0.0039 mm Particles < 1/256 mm, or 0.0039 mm A group of sheet silicate minerals (not micas) that are commonly clay-sized A group of sheet silicate minerals (not micas) that are commonly clay-sized Original description from not being able to identify small grain size material Original description from not being able to identify small grain size material Now can use X-ray diffraction to determine clays Now can use X-ray diffraction to determine clays

33. Terminology Clay: Sediment composed of particles that are < 0.002 mm Clay: Sediment composed of particles that are < 0.002 mm Claystone: Rock composed of clay-sized particles Claystone: Rock composed of clay-sized particles Clay minerals: 1:1 and 2:1 Phyllosilicate minerals without K + or Ca 2+ bonding sheets (those are Micas) Clay minerals: 1:1 and 2:1 Phyllosilicate minerals without K + or Ca 2+ bonding sheets (those are Micas) Argillaceous: Rock or sediment containing large amounts of clay and clay minerals Argillaceous: Rock or sediment containing large amounts of clay and clay minerals

34. Problems Problems Clay size fraction can contain other minerals (quartz, carbonates, zeolites etc.) Clay size fraction can contain other minerals (quartz, carbonates, zeolites etc.) “Clay” used to define size fraction – size not mineralogical “Clay” used to define size fraction – size not mineralogical Several clay minerals can be larger than the size requirements Several clay minerals can be larger than the size requirements

35. Clay classification 1:1 layer clays 1:1 layer clays 7 Å type 7 Å type TO layers TO layers Kaolinite (dioctahedral) Kaolinite (dioctahedral) Serpentine (trioctahedral) Serpentine (trioctahedral)

36. 2:1 layer clays: 2:1 layer clays: End members: End members: 10 Å – Pyrophyllite (dioctahedral) & talc (trioctahedral) 10 Å – Pyrophyllite (dioctahedral) & talc (trioctahedral) 10 Å – Charge imbalance: with K as interlayer: Mica: Muscovite (trioctahedral) & Biotite (dioctahedral) 10 Å – Charge imbalance: with K as interlayer: Mica: Muscovite (trioctahedral) & Biotite (dioctahedral) 14 Å type (Chlorite) 14 Å type (Chlorite)

37. Intermediate 2:1 clays Intermediate 2:1 clays Have net negative charge, but less than one per formula Have net negative charge, but less than one per formula Requires less interlayer cations to charge balance Requires less interlayer cations to charge balance Mixed layer clays – combined 1:1 and 1:2 Mixed layer clays – combined 1:1 and 1:2

38. Three types of intermediate 10 Å clays Three types of intermediate 10 Å clays Low charge imbalance – smectite clays Low charge imbalance – smectite clays High charge imbalance – illite clays High charge imbalance – illite clays Intermediate charge imbalance – vermiculite Intermediate charge imbalance – vermiculite Charge imbalance controlled by Charge imbalance controlled by “interlayer” cations “interlayer” cations They move in and out – Cation Exchange Capacity (CEC) They move in and out – Cation Exchange Capacity (CEC) Surface adsorption Surface adsorption

39. Low charge Smectite Smectite approximately = Ca 0.17 (Al,Mg,Fe) 2 (Si,Al) 4 O 10 (OH) 2 nH 2 O approximately = Ca 0.17 (Al,Mg,Fe) 2 (Si,Al) 4 O 10 (OH) 2 nH 2 O Net negative charge is 0.2 to 0.6 per formula unit, typically 0.33 Net negative charge is 0.2 to 0.6 per formula unit, typically 0.33 Ca and Na are typical interlayer ions Ca and Na are typical interlayer ions Exchangable Exchangable May be dioctahedral or trioctahedral May be dioctahedral or trioctahedral Charge results from Charge results from Al substitution for Si in tetrahedron Al substitution for Si in tetrahedron Mg for Al in octahedron (in dioctahedral) Mg for Al in octahedron (in dioctahedral)

40. Low charge means water and cations (Na, K, Ca, Mg) easily move in and out of interlayer sites Low charge means water and cations (Na, K, Ca, Mg) easily move in and out of interlayer sites No water = 10 Å No water = 10 Å One water layer = 12.5 Å One water layer = 12.5 Å Two water layer = 15.2 Å Two water layer = 15.2 Å Water moves in and out depending on moisture in environment Water moves in and out depending on moisture in environment

41. High charge Illite/glauconite Illite/glauconite Approximately = K 0.8 Al 2 (Al 0.8 Si 3.2 )(OH) 2 Approximately = K 0.8 Al 2 (Al 0.8 Si 3.2 )(OH) 2 Net negative charge of 0.8 to 1 per formula Net negative charge of 0.8 to 1 per formula Very similar to muscovite – called mica-like Very similar to muscovite – called mica-like Mostly substitute of Al 3+ for Si 4+ Mostly substitute of Al 3+ for Si 4+ All are dioctahedral; Glauconite has Fe 3+ All are dioctahedral; Glauconite has Fe 3+ Interlayer ion is K + Interlayer ion is K + High K concentration means strong bond High K concentration means strong bond Difficult for water to enter Difficult for water to enter Non-swelling clay Non-swelling clay

42. Intermediate charge Vermiculite Vermiculite Approximately = (Mg,Ca) 0.3 (Mg,Fe 2+,Fe 3+,Al) 3 (Si,Al) 4 )O 10 (OH) 2 Approximately = (Mg,Ca) 0.3 (Mg,Fe 2+,Fe 3+,Al) 3 (Si,Al) 4 )O 10 (OH) 2 About 0.6 charge per formula unit About 0.6 charge per formula unit Comes from oxidation of Fe 2+ to Fe 3+ in biotite Comes from oxidation of Fe 2+ to Fe 3+ in biotite Reduces the negative charge on TOT layer from -1 to -0.6 Reduces the negative charge on TOT layer from -1 to -0.6 Less K + than mica, can exchange for Ca 2+ and Mg 2+ and water Less K + than mica, can exchange for Ca 2+ and Mg 2+ and water Swell clay Swell clay With water interlayer spacing is 14.4 Å With water interlayer spacing is 14.4 Å

43. Mixed layer clays Natural clays rarely similar to the end members Natural clays rarely similar to the end members Typically contain parts of different types of clays Typically contain parts of different types of clays Actually mixtures at unit cell level – not physical mixtures Actually mixtures at unit cell level – not physical mixtures Nomenclature – combined names Nomenclature – combined names Illite/smectite or chlorite/smectite Illite/smectite or chlorite/smectite

44. 1:1 layer clays 2:1 layer clays – low charge, smectite 2:1 layer clays – high charge, illite 2:1 layer clays – Chlorite gp Mixed layer 7 Å 10 Å 14 Å Figure 13-15

45. Burial Diagenesis Smectite converts to illite with burial Smectite converts to illite with burial Most conversion at 50 to 100º C Most conversion at 50 to 100º C Conversion requires K, usually comes from dissolution of K spar Conversion requires K, usually comes from dissolution of K spar

46. Mineralogy of Miocene/Oligocene sediments Gulf Coast 1.K-spar dissolves 2.Smectite converts to illite (with extra K) 3.Releases interlayer water 4.Increase pore pressures 5.T corresponds to “oil window” 6.Forces oil from pore spaces into reservoirs Figure 13-16